Electrochemical cells having low vapor pressure complexed SO2 electrolytes

ABSTRACT

A non-aqueous electrochemical cell having a solid active cathode, an active metal anode and a low vapor pressure highly conductive electrolyte comprising a liquid solvate-complex of sulfur dioxide (SO 2 ) and an alkali or alkaline earth metal salt soluble therein such as those having a Group 3A element halide anion with the ratio of salt to SO 2  in said electrolyte ranging from about 1:1 to 1:7.

This application is a continuation of application Ser. No. 405,980, filed Aug. 9, 1982, now abandoned.

This invention relates to electrolytes for non-aqueous electrochemical cells and more particularly to such electrolytes containing sulfur dioxide (SO₂).

Sulfur dioxide, though a poor electrolyte solvent has nevertheless been widely utilized in non-aqueous electrochemical cells in the dual function of cathode depolarizer and electrolyte solvent because of its high energy density and high rate capability. Without its functioning as the cathode depolarizer SO₂ has rarely been utilized as an electrolyte solvent alone since in addition to its poor solvating properties SO₂ has several other shortcomings which are generally related to its being a gas at room temperature and pressure (B.P. 10° C.). For proper utilization the gaseous SO₂ is converted into a liquid under conditions of low temperature and/or high pressure and must be maintained in such liquid form by constant pressurization. As a result, cells having SO₂ therein have had the attendant disadvantage of requiring expensive reinforced cell containers and hermetic seals resistant to the volatile SO₂. Additionally, expense was further incurred as a result of the need for the initial liquefaction of the SO₂ and the special handling required particularly with respect to the filling of the cells with the volatile liquid SO₂. The safety aspects of cells containing SO₂ were also somewhat of a problem since required safety venting mechanisms, while providing protection, nevertheless operated by the atmospheric expulsion of noxious gaseous SO₂.

In order to aid in electrolyte solvation and to reduce the high vapor pressure of the SO₂, cells have generally contained organic cosolvents such as acetonitrile with the SO₂. However, despite the presence of the vapor pressure reducing organic cosolvents, the cells nevertheless remained highly pressurized with its attendant disadvantages. Furthermore, organic cosolvents such as the aforementioned acetonitrile generally precluded efficient cell cycling and occasionally in themselves posed potential saftey problems when the cells were abused. Electrolyte salts which were found to readily dissolve in the SO₂ without the necessity for organic cosolvents either provided poorly operating cells e.g. LiAlCl₄ or were generally prohibitively expensive e.g. clovoborate salts such as Li₂ B₁₀ Cl₁₀. Furthermore, such salts when dissolved in the liquified SO₂ though alleviating hazard problems associated with organic co-solvents did not alleviate the problems caused by the high vapor pressure of the sulfur dioxide.

It is an object of the present invention to provide a non-aqueous cell having a low vapor pressure electrolyte based upon SO₂ and a method for the preparation of such electrolyte.

It is a further object of the present invention to provide such electrolyte having a very high conductivity and thus being suitable for high rate applications; being economical with respect to both component materials annd the preparation thereof; being a liquid at normal temperature and pressure; being chemically stable in non-aqueous cell environments; being suitable for secondary or rechargeable cell applications; being suitable as an electrolyte over a wide temperature range; and being without free organic materials and therefore being safer than conventional organic electrolytes.

These and other objects, features and advantages of the present invention will become more evident from the following discussion and drawings in which:

FIG. 1 is a vapor pressure comparison of a prior art SO₂ electrolyte and an electrolyte of the present invention,

FIG. 2 is a graph of the conductivities of various electrolytes of the present invention at various temperatures;

FIG. 3 is a graph of the cycling characteristics of a cell having the electrolyte of the present invention;

FIGS. 4-7 are discharge curves of various cells having the electrolyte of the present invention. FIG. 5 also depicts a charging curve for one embodiment; and

FIG. 8 is a graph of the polarization characteristics of a cell having an electrolyte of the present invention.

Generally the present invention comprises a low vapor pressure (below 2 atm at room temperature and preferably below 1 atm) high conductivity electrolyte based upon SO₂ ; a method for the preparation thereof; and both non-aqueous primary and secondary cells having active metal anodes such as of alkali or alkaline earth metal including mixtures and alloys thereof and containing such low vapor pressure electrolyte. The electrolyte of the present invention is comprised of a tightly bound solvate-complex of SO₂ and an alkali or alkaline earth metal salt soluble therein such as those wherein the anion of the salt is comprised of a Group 3A (of the Periodic Table) element halide. The Group 3A elements which are preferred for the salt are boron, aluminum, gallium and indium and the preferred alkali and alkaline earth metals are lithium, sodium and calcium. Examples of preferred Group 3A salts suitable for complexing with the SO₂ include LiAlCl₄, LiGaCl₄, LiBF₄, LiBCl₄, LiInCl₄, NaAlCl₄, NaGaCl₄, NaBF₄, NaBCl₄, NaInCl₄, Ca(AlCl₄ )₂, Ca(GaCl₄)₂, Ca(BF₄)₂ Ca(BCl₄)₂, Ca(InCl₄)₂, Sr(AlCl₄)₂, Sr(GaCl₄)₂, Sr(BF₄)₂, Sr(BCl₄)₂, Sr(InCl₄)₂ and mixtures thereof. Non-Group 3A salts suitable for complexing with the SO₂ include Li₃ SbCl₆ and LiSbCl₆. Salts which do not complex with the SO₂ may however be dissolved in the solvate-complex if desired.

The solvate-complexing of the SO₂ and the salt is dependent upon equivalent ratios of the materials rather than mole ratios with such difference being apparent with respect to, for example, alkaline earth metal salts which generally contain two equivalents per mole. The equivalent ratios range from about 1:1 to 1:4 (salt:SO₂) and because of such variation the combination of the salt and SO₂ is demonstrated as being in the nature of a complex rather than a reaction-formed new compound. Though the actual complexing of the salt to SO₂ is generally up to a ratio of 1:4 (salt:SO₂), addition of uncomplexed SO₂ to the cell in an amount of up to about 1:7 (salt:total SO₂) will not generally detrimentally pressurize the cell at room temperature. With salt to SO₂ ratios above 1:7 not only does the cell become detrimentally pressurized (above about 2 atm.), the conductivity of the electrolyte is also reduced thereby. Electrolytes such as the 1 molar LiAlCl₄ in SO₂ (equivalent ratio of about 1:22 of LiAlCl₄ :SO₂) as described in U.S. Pat. No. 3,493,433 are highly pressurized (about 3.5 atm.) and are in fact described in said patent as being utilized at temperatures between -10° C. to -30° C. Because of its being in unavailable complexed form with only a minimal, if any, content of uncomplexed SO₂, the SO₂ in the electrolyte does not function as the active cathode depolarizer. The electrolyte of the present invention therefore has its main utility in electrochemical cells having solid cathode depolarizers. Such cathodes include CuCl₂, CuO, CuS, MnO₂, Cr₃ O₈, V₂ O₅ as well as other metal halides, oxides, chromates, vanadates, titanates, tungstates, chalcogenides and active non-metal cathodes such as organic conductive polymers such as polyacetylene, poly-p-phenylene, polyphenylene sulfide and various carbon compounds such as C_(x) S and CF_(n).

Though it would have been expected that the solid cathodes would provide cells having reduced high rate capability when compared to cells having fluid cathode depolarizers such as SO₂ such reduction is in fact minimized by the unexpectedly very high conductivity of the electrolyte of the present invention. Furthermore, advantages of a substantially non-pressurized system, particularly with respect to increased safety more than compensates for any reduction in high rate capability.

The solvate-complex electrolyte of the present invention is prepared by reacting the SO₂ with the alkali or alkaline earth metal salt in the requisite equivalent ratios. Such reaction may be effected by substantially saturating liquified SO₂ with the salt to the requisite equivalent ratios. It is however preferred that the salt be reacted with the SO₂ in gaseous form such as by passing a stream of dry SO₂ through the salt whereby an exothermic reaction occurs with the formation of a liquid solvate-complex. The resultant liquid has a low vapor pressure (<10 psi at 20° C. with a B.P. of about 40° C.) and can be handled as a liquid in contrast to liquified SO₂ (B.P. 10° C.) which must be specially handled as a volatile material. FIG. 1 compares the vapor pressure of prior art SO₂ containing electrolyte (Curve A) and a LiAlCl₄.3.5SO₂ solvate-complex (Curve B) at various temperatures. At room temperature (20° C.) prior art SO₂ electrolytes have high vapor pressures (about 50 psi or about 31/2 atm) whereas the solvate-complex has a vapor pressure of about 10 psi, well below atmospheric pressure of 15 psi. Pressure rise of the solvate-complex electrolyte is logarithmical with a rise in temperature to a pressure of 60 psi at 80° C. This is in further marked contrast to the 260 psi at 80° C. of the prior art SO₂ -containing electrolytes.

It is noted that the SO₂ need not be directly reacted with the salt per se but may in fact be reacted with for example the Lewis acid and base components of the salt whereby the salt and the liquid solvate-complex are simultaneously produced. For example, a stream of dry SO₂ may be passed through a LiAlCl₄ salt or a 1:1 stoichiometric mixture of the Lewis acid and base components thereof, LiCl and AlCl₃, to produce the same liquid LiAlCl₄.xSO₂ solvate-complex, with "x" having been determined as ranging from about 1 to 4 (equivalent basis). Continued bubbling of SO₂ therethrough results in a higher value for "x" in the range. Lower values for "x" may be obtained by evaporating some of the SO₂ from the liquid solvate-complex. Addition of excessive SO₂ as described above wherein the ratio of total SO₂ to salt exceeds 7:1, however, forms an undesirable pressurized (above about 2 atm) electrolyte.

The conductivity at room temperature of a solvate-complex electrolyte of LiAlCl₄.3.5SO₂ has been discovered to be about 1×10⁻¹ ohm⁻¹ cm⁻¹ which is the highest conductivity observed to date in any non-aqueous electrolyte. The electrolyte of the present invention has been found to be very stable with lithium anodes and has been further found to enable lithium to be electrochemically plated and stripped therein with efficiencies of over 97% even over extended cycling regimens whereby it is an eminently suitable electrolyte for rechargeable lithium or other alkali or alkaline earth metal cells.

A solvate-complex of NaAlCl₄.2.8SO₂ while having somewhat lower conductivity of 8×10⁻² ohm⁻¹ cm⁻¹ (but still very high) has the advantage of good low temperature operability e.g. conductivity of 2×10⁻² ohm⁻¹ cm⁻¹ even at -30° C. In contrast to the LiAlCl₄.xSO₂ solvate-complexes which freeze at temperatures between about -8° C. to -15° C. the NaAlCl₄.xSO₂ solvate-complexes freeze at about -44° C. and are more suitable when low temperature operation is desired. A solvate-complex of Ca(AlCl₄.2.75SO₂)₂ which has a conductivity of 1.7×10⁻² ohm⁻¹ cm⁻¹ at room temperature and about 10⁻³ ohm⁻¹ cm⁻¹ at -35° C. does not in fact freeze but rather becomes immobile at about -50° C. It can accordingly be admixed with other solvate-complex salts such as LiAlCl₄.3.5SO₂ to provide an electrolyte having both high conductivity and extremely low and high temperature capability.

Alternatively, it has been discovered that the incorporation of additive amounts of inorganic solvents such as SOCl₂, S₂ Cl₂, SCl₂, SO₂ Cl₂ admixures thereof to the solvate-complex electrolyte also serves to enhance low temperature capability. For example, a mixture of 90% (by weight) LiAlCl₄.2.7SO₂ and 10% SOCl₂ (containing 1M LiAlCl₄) freezes at about -25° C. with a conductivity of about 1.8×10⁻² ohm⁻¹ cm⁻¹ at -20° C.

Though lithium has been found to be stable in the presence of, for example, the NaAlCl₄ and Ca(AlCl₄)₂ solvate-complex electrolytes, in secondary or rechargeable cell applications it is preferred that the complexed salt contains cations corresponding to the anode metal.

For primary cell applications other salts such as the aforementioned sodium and calcium salts may be affectively utilized with lithium anodes particularly in providing enhanced low temperature capability. Additionally and preferably for primary cell applications electrolyte salts normally not soluble in SO₂ alone may be utilized by stoichiometric complexing with both SO₂ and an organic cosolvent such as acetonitrile; ethers such as dimethoxyethane; propylene carbonate and the like. Such salts include LiBr, LiClO₄, LiAsF₆ and LiPF₆. The organic cosolvent thereby make such salts soluble in the SO₂. The organic cosolvent is present only in sufficient quantity to co-complex the salt with the hazards of free organic materials not being present. Soluble salts may similarly be utilized with such co-complex.

The fact that the electrolyte of the present invention has a low vapor pressure despite its SO₂ component results in several very important economic and safety benefits. Cells made therewith need not be reinforced or otherwise made resistant to pressurized contents. Venting, if at all necessary as a safety precaution does not result in the emission of rapidly spreading noxious SO₂ fumes. Hermetic seals for the cells are not as susceptible to degradation because of the bound state of the normally corrosive SO₂ and in fact more economical seals suitable for non-pressurized cells may be utilized. Except for the initial relatively simple procedure of forming the liquid solvate-complex electrolyte as described above no special handling or storage is required in contrast to the handling of uncomplexed volatile SO₂. Filling of cells with the electrolyte is simply with a stable liquid as opposed to filling cells with a volatile pressurized liquid such as SO₂.

In order to more fully illustrate the properties and beneficial aspects of the electrolyte of the present invention the following examples are presented. It is understood, however, that such examples are illustrative in nature and are not to be construed as limiting the present invention. Unless otherwise indicated all parts are parts by weight.

EXAMPLE 1

Stoichiometric amounts of LiCl and AlCl₃ were placed in a glass vessel and dry SO₂ was passed through the salt particles. A clear liquid solvate-complex of LiAlCl₄.SO₂ was formed rapidly with the generation of heat and after cooling to room temperature the molar or equivalent ratio of LiAlCl₄ to SO₂ in the solvate-complex was determined to be 1:3.1. Continued bubbling of the dry SO₂ through the LiCl and the AlCl₃ provided another clear liquid solvate-complex having an equivalent ratio of LiAlCl₄ to SO₂ of 1:3.5. Evaporation of some SO₂ from the LiAlCl₄.3.1SO₂ solvate-complex provided another clear liquid solvate-complex having an equivalent ratio of LiAlCl₄ to SO₂ of 1:2.6. The conductivities of the three liquid solvate-complexes at various temperatures were measured as shown in FIG. 2 as curves C, D and E respectively. The conductivities obtained were the highest ever obtained for nonaqueous electrolytes. Additionally, lithium metal stored in the electrolytes for periods in excess of four weeks showed no corrosion thereby indicating the stability of such solvate-complex electrolytes in lithium containing cells.

EXAMPLE 2

A NaAlCl₄.2.8SO₂ solvate-complex was formed as in Example 1 but with NaCl instead of LiCl with its conductivity at various temperatures shown in FIG. 2 as Curve F.

EXAMPLE 3

A Ca(AlCl₄.2.75SO₂)₂ -equivalent ratio (Ca(AlCl₄)₂.5.5SO₂ molar ratio) solvate-complex was formed as in Example 1 but with a stoichiometric ratio of CaCl₂ :2AlCl₃. The conductivity at various temperatures is shown in FIG. 2 as Curve G.

EXAMPLE 4

A mixed solvate complex of (LiAlCl₄ +4Ca(AlCl₄)₂).5.6SO₂ (molar ratio) was made by passing SO₂ through AlCl₃, LiCl and CaCl₂ (molar ratio of 9:1:4). The resultant liquid solvate-complex provided conductivities at various temperatures as shown in FIG. 2 as Curve H.

EXAMPLE 5

A mixture of 90% LiAlCl₄.2.7SO₂ solvate-complex and 10% 1M LiAlCl₄ in SOCl₂ was made and provided conductivities at various temperatures as shown in FIG. 2 as Curve I.

EXAMPLE 6

A cathode limited electrochemical cell was constructed with a 2 gram cathode of 60% CuCl₂, 30% graphite and 10% polytetrafluoroethylene (PTFE) pressed on a nickel grid, 1" (2.5 cm)×1.6" (4.1 cm) sandwiched by two lithium anodes each of similar dimensions. The cell was filled with the LiAlCl₄.3.1SO₂ solvate-complex electrolyte of Example 1 and discharged at the rate of 40 ma (2 ma/cm²) to a 2.6 volt cutoff and charged at a rate of 20 ma (1 ma/cm²) to 4.05 volts on a continuous cycling regimen. The cell has cycled for about 350 cycles at close to 100% of the one electron transfer cathode capacity (Li+CuCl₂ →CuCl+LiCl). The charging and discharging curves for cycles nos. six, 173 and 230 are shown in FIG. 3. Cumulative capacity so far is 72 Ahrs with about 36 Li turnovers (97% eff. on the anode). The initial theoretical primary capacity was 0.24 Ahr.

EXAMPLE 7

Three cells were made as in Example 6 but with the solvate-complex electrolyte, (LiAlCl₄ +4Ca(AlCl₄)₂).5.6SO₂ of Example 4. The cells were discharged at rates of 20 ma, 40 ma and 60 ma respectively with discharge results as shown in FIG. 4.

EXAMPLE 8

A cell as in Example 6 was made but with the solvate complex electrolyte of Example 5 (90% LiAlCl₄.2.7SO₂ and 10% 1M LiAlCl₄ in SOCl₂). The cell was cycled at 40 ma (2 ma/cm²) discharge 20 ma (1 ma/cm²) charge and the discharge-charge of the sixth cycle is shown in FIG. 5.

EXAMPLE 9

A cell as in Example 6 was made but with a 3 gram cathode comprised of 60% CuO, 30% graphite and 10% PTFE. The cell was discharged at 40 ma with results as shown in FIG. 6.

EXAMPLE 10

A cell as in Example 6 was made but with a 3 gram cathode comprised of 60% CuS, 30% graphite and 10% PTFE. The cell was discharged at 40 ma with results as shown in FIG. 7.

EXAMPLE 11

A spirally wound Li/MnO₂ cell having electrodes 6.25" (15.9 cm)×1" (2.5 cm) of lithium and MnO₂ with the cell being cathode limited to a theoretical capacity of 0.75 Ahrs was filled with the LiAlCl₄.3.1SO₂ electrolyte of Example 1. The cell showed an initial open circuit voltage of 4.0 volts. The cell was then discharged at 85 ma and delivered a capacity of 0.56 Ahr to a voltage cutoff of 2.0 volts. The cell was charged at 40 ma for 14 hours and discharged again at 40 ma delivering 0.40 Ah on the second discharge. FIG. 8 shows the polarization of the cell during discharge and charge.

Effective utilization of the low vapor pressure electrolyte of the present invention in Li/MnO₂ cells may alleviate problems relating to the reactive electrolyte salts and volatile solvents generally utilized in such electrochemical cell systems such as lithium perchlorate and dimethoxyethane by simply replacing them with the relatively safe electrolyte of the present invention.

From the above examples it is evident that the solvate-complex electrolytes of the present invention provide for substantially non-pressurized cells which are efficiently rechargeable and that some embodiments have conductivities well above those of previously known non-aqueous electrolytes which did not generally have room temperature conductivities above about 5×10⁻² ohm⁻¹ cm⁻¹.

It is understood that the above examples are illustrative in nature and that changes in the cathode materials, electrolyte composition and ratios as well as the cell systems in which they are utilized may be made without departing from the scope of the present invention as defined in the following claims. 

What is claimed is:
 1. A non-aqueous, electrochemical cell having an active metal anode, a solid active cathode and a liquid electrolyte characterized in that said electrolyte consists essentially of a low vapor pressure, liquid solvate-complex of (a) sulfur dioxide annd (b) one or more alkali or alkaline earth metal salts soluble in said sulfur dioxide and also capable of forming a complex therewith, wherein said one or more salts consists essentially of a stoichiometric reaction product of a Lewis acid and a Lewis base, with the equivalent ratio of said one or more salts to SO₂ in said liquid electrolyte ranging from 1:1 to 1:7, and wherein all of said one or more alkali or alkaline earth metal salts is complexed with said sulfur dioxide.
 2. The cell of claim 1 wherein said one or more salts contains Group 3A element halide anions.
 3. The cell of claim 1 wherein one or more salts are selected from the group consisting of LiAlCl₄, LiGaCl₄, LiBF₄, LiBCl₄, LiInCl₄, NaAlCl₄, NaGaCl₄, NaBF₄, NaBCl₄, NaInCl₄, Ca(AlCl₄)₂, Ca(GaCl₄)₂, Ca(BF₄)₂, Ca(BCl₄)₂, Ca(InCl₄)₂, Sr(AlCl₄)₂, Sr(GaCl₄)₂, Sr(BF₄)₂, Sr(BCl₄)₂, Sr(InCl₄)₂, Li₃ SbCl₆, LiSbCl₆ and mixtures thereof.
 4. The cell of claim 1 wherein said solid active cathode is comprised of a material selected from the group consisting of metal halides, oxides, chromates, vanadates, titanates, tungstates, and chalcogenides, polyacetylene, poly-p-phenylene, polyphenylene sulfide, C_(x) S and Cf_(n).
 5. The cell of claim 1 wherein said active metal anode is comprised of lithium.
 6. The cell of claim 1 wherein said equivalent ratio ranges from 1:1 to 1:4.
 7. The cell of claim 1 wherein said electrolyte is further comprised of an organic electrolyte which with said SO₂ is co-complexed with said one or more salts.
 8. The cell of claim 1 wherein said cell further includes a minor amount of an inorganic additive selected from the group consisting of SOCl₂, SO₂ Cl₂, S₂ Cl₂, SCl₂ and mixtures thereof.
 9. A low vapor pressure, non-aqueous electrochemical cell comprising a lithium anode, a solid active cathode and a liquid electrolyte consisting essentially of SO₂ complexed with one or more salts selected from the group consisting of LiAlCl₄, NiAlCl₄ and Ca(AlCl₄)₂ wherein the equivalent ratio of said one or more salts to SO₂ in said liquid electrolyte ranges from 1:1 to 1:4, with said one or more salts consisting essentially of stoichiometric amounts of LiCl, NaCl and CaCl₂ with AlCl₃ respectively, and wherein all of said one or more salts is complexed with said sulfur dioxide.
 10. The low vapor pressure electrochemical cell of claim 9 wherein said solid active cathode is comprised of a member selected from the group consisting of CuCl₂, CuO, CuS and MnO₂.
 11. The cell of claim 10 wherein said solid active cathode is comprised of CuCl₂.
 12. A non-aqueous electrochemical cell comprising a lithium anode, a CuCl₂ cathode and a liquid electrolyte consisting essentially of SO₂ complexed with LiAlCl₄. 